In the proposed studies, new 211At-labeled reagents will be developed for application to therapy of metastatic prostate cancer. The new reagents will target the prostate specific membrane antigens (PSMA) or gastrin-releasing peptide receptors (GRP-R) on human prostate cancer cells. The studies will evaluate the in vitro binding and internalization of the new agents in C4-2 and PC-3 human prostate cancer cells. The studies will also evaluate tumor localization, pharmacokinetics and tissue distributions in SCID mice bearing C4-2 human prostate cancer xenografts, and evaluate the efficacy and toxicity of the best candidates in a quasi-metastatic bone metastases model in SCID mice. Targeting of the prostate cancer cells will be achieved by using an anti-PSMA antibody Fab4, the GRP-R-binding peptide bombesin, or a high binding affinity inhibitor of the enzymatic activity of PSMA. Because all three of these agents suffer from localization in kidney, protein-based conjugates will be prepared that have physical sizes and charges that exclude them from that organ. In specific aim 1 (SA 1), the antibody Fab4 fragment will be conjugated with four molecules of differing size and charge that contain the decaborate(2-) and branched-chain discrete-sized polyethyleneglycol (dPEG) moieties. In specific aim 2, bombesin or a PSMA inhibitor will be combined with decaborate(2-) in a trifunctional reagent. Those trifunctional molecules will be conjugated with recombinant streptavidin (rSAv) and that conjugate will be succinylated to demonstrate cancer targeting in that model system. In specific aim 3, the same molecules used in SA1 and SA2 will be conjugated with recombinant human serum albumin (rHSA) to provide 211At-labeled reagents that will be excluded from kidney, but will also have low immunogenicity. In specific aim 4, the three 211At-labeled reagents from SA1-3 that have the most favorable properties will be evaluated for efficacy and toxicity in a SCID mouse model for prostate cancer metastatic to bone. From the research effort one or two new 211At-containing reagents will be identified as having promise for more extensive evaluation in treatment of metastatic prostate cancer.